Khandogya-Upanishad: Fourth Prapathaka, Sixteenth Khanda
Because moving along he purifies everything, therefore he is the sacrifice. Of that sacrifice there are two ways, by mind and by speech.
2. The Brahman priest performs one of them in his mind, the Hotri, Adhvaryu, and Udgatri priests perform the other by words. When the Brahman priest, after the Pritaranuvaka ceremony has begun, but before the recitation of the Paridhaniya hymn, has (to break his silence and) to speak,
3. He performs perfectly the one way only (that by words), but the other is injured. As a man walking on one foot, or a carriage going on one wheel, is injured, his sacrifice is injured, and with the injured sacrifice the sacrificer is injured; yes, having sacrificed, he becomes worse.
4. But when after the Pritaranuvaka ceremony has begun, and before the recitation of the Paridhaniya hymn, the Brahman priest has not (to break his silence and) to speak, they perform both ways perfectly, and neither of them is injured.
5. As a man walking on two legs and a carriage going on two wheels gets on, so his sacrifice gets on, and with the successful sacrifice the sacrificer gets on; yes, having sacrificed, he becomes better.